Remote detection of explosive substances

ABSTRACT

Apparatus and methods for effectively detecting and locating explosive substances within remote targets, including improvised explosive devices (IEDs). The detection apparatus includes a neutron beam generator, a gamma ray detector, data collection modules and sensors, and a detection processing module. The neutron beam generator includes a fast neutron source, a neutron moderator to slow some or all of the fast neutrons to thermal energies, a partially enclosing neutron shield, and a rotatable neutron shield surrounding the generated neutrons. The neutron shield has an aperture to form a neutron beam directed at a remote target. If the remote target contains explosive substances, gamma rays radiate isotropically from the remote target when it is bombarded by the neutrons. A portion of these gamma rays are intercepted and detected by the gamma ray detector, which is spaced apart from the neutron source. The detection processing module determines whether the remote target contains explosive substances and further locates the target by processing the collected data from the gamma ray detector, status information collected from the neutron source, and the position sensor(s) associated with the neutron shield.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of patent application Ser.No. 11/489,261 filed Jul. 18, 2006 and claims priority to ProvisionalPatent Application No. 61/168,244 filed Apr. 10, 2009, the entiredisclosures of which are hereby incorporated by reference and reliedupon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates generally to the detecting of explosivesubstances, and more particularly to the detecting of explosivesubstances within remote targets and the locating of such targets.

2. Related Art

An improvised explosive device (IED) is an explosive device placed orfabricated in an improvised manner, often used in unconventional warfareby terrorists or guerrillas. These IEDs are sometimes referred to asroadside or car bombs. The ever-increasing need to protect soldiers andcivilians alike has resulted in demand for explosives detection systemsthat can detect and locate an IED at a significant standoffdistance—ideally, near or beyond the IED's kill radius.

It is well known that explosives can be detected by bombarding them withthermal or slow neutrons of kinetic energy levels of approximately 0.026eV, then detecting the resulting gamma rays. The vast majority ofconventional chemical explosives are nitrogen-14 (14N) rich, whileSpecial Nuclear Materials (SNMs) may contain Plutonium-239 (239Pu),Uranium-235 (235U), or, both as key ingredients. Each of theseconstituent elements, 14N, 239Pu, and 235U, as well as other materialsof interest in detecting improvised explosives, for example, chlorine,copper, and aluminum, among others, radiates its own characteristicgamma ray emission spectrum when dosed with thermal neutrons.

For instance, militarily significant conventional chemical explosives,which constitute by far the largest threat to human life in terms of thefrequency of occurrence, historical lethality, and ease of procurementand use, contain very high densities of nitrogen, principallynitrogen-14. Nitrogen-14, when bombarded by a thermal neutron, emits astrong gamma ray with energy 10.83 MeV as follows:¹⁴N+¹ n→ ¹⁵N+γ  [gamma ray]

The gamma ray emission is isotropic in that it can be emitted in anydirection, and its trajectory is uncorrelated to the trajectory of theincident thermal neutron. High gamma ray fluxes are interpreted asexplosives detection events. This technique is known as Thermal NeutronActivation Analysis (TNAA).

TNAA is a well-known technique for explosives detection and other typesof materials analysis. However, the majority of TNAA technology has beendirected at explosives detection in luggage and at landmine detection.Both applications operate in environments with complicating factors thatlimit the success of TNAA. Many common items found in luggage, such asnylon sweaters, are rich in nitrogen. This reduces the signal-to-noiseratio (SNR), which increases the false alarm rate and lowers the overalldetection rate. Explosives distributed in small pieces in luggage alsoreduce the SNR and the detection rate in TNAA. Likewise, the mostsignificant issue with landmine and buried explosives detection is thepresence of significant amounts of silicon-29, which constitutes up to5% of native silicon, and which emits gamma rays at 10.6 MeV underthermal neutron dosing. This emission (noise) competes with the gammarays from nitrogen at 10.83 MeV (signal), reducing the SNR, increasingthe false alarm rate, and decreasing the overall detection rate.Furthermore, the reduced SNR in both applications translates intoincreased inspection times and decreased throughout.

By contrast, in accordance with embodiments of the present invention,the proposed use of TNAA for IED detection operates in more conduciveenvironments. First, the most deadly IEDs contain significant amounts ofexplosives, and hence, of nitrogen, since they are very large comparedto anti-personnel land mines or to bombs in luggage. Thus, the targetedsignal is high compared to competing signals from other noise sourcesfrom environmental nitrogen-14 and silicon-29. This has the effect ofimproving the SNR, decreasing the false alarm rate, and increasing theoverall detection rate. Second, IEDs are often placed at or above theground or buried with shallow overburden when compared to theirexplosive weight; they are often buried in trash piles or placed nearconcrete or dirt roads. Although silicon is present in theseenvironments, its effect is significantly smaller than that in the caseof buried antipersonnel mines with amounts of explosives measured inounces. A typical IED is a command detonated device whose primarycomponent is one or more HE (high explosive) 155 mm (U.S./NATO) or 152mm (Soviet/WTO/Indigenous copy) (diameter) artillery rounds consistingof a metal casing filled with high explosive and measuring about 450 mmlong. Increasingly, so-called “home made explosives” [HMEs] are alsofound, often consisting of ammonium nitrate plus fuel oil, urea nitrate,and similar nitrogen-rich compounds.

More importantly, there is presently no device for effectively detectingand locating IEDs at a standoff distance. Landmine detection andexplosives detection in luggage both examine targets at close proximity.Therefore, a further object of the present invention is to detect IEDsusing TNAA under different conditions, and thereby significantly reducefriendly and civilian casualties. Still further objects and advantageswill become apparent from a consideration of the ensuing description anddrawings.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and methods foreffectively detecting and locating explosive substances within remotetargets, such as an IED including an artillery round, but not solimited. One major advantage afforded by embodiments of the presentinvention is that IEDs, including roadside and car bombs, may bedetected and located at a standoff distance (e.g. at least 5 meters, 10meters, or further), thereby reducing casualties and deterring futureIED attacks, especially in civilian areas. For instance, a portabledetection apparatus in accordance with the present invention may bemounted on a vehicle such that explosive materials could be identifiedsafely and effectively on a routine patrol. (Although a standoffdistance of 10 meters would not be sufficient to prevent the death of adismounted soldier, it would afford significant protection to a soldierinside a modern blast-resistant military vehicle.)

Briefly, the disclosed detection apparatus includes a thermal neutronbeam generator, a gamma ray detector, a plurality of data collectionmodules and sensors, and a detection processing module. The thermalneutron beam generator comprises a fast neutron source, a neutronmoderator to slow some or all of the fast neutrons to thermal energies,and a rotatable neutron shield enclosing the generated thermal neutrons.The neutron shield has an aperture to form a thermal neutron beam thatmay be directed at or scanned over a remote target. If the remote targetcontains explosive substances or other items of interest, gamma raysradiate isotropically from the remote target when it is bombarded by theneutrons. A portion of these gamma rays are intercepted and detected bythe gamma ray detector, which is spaced a few meters apart from thethermal neutron source in order to minimize the nuisance signals createdby the neutron-irradiated air path seen by the detector, as well as bythe neutron source itself, thereby reducing background noise. Thisarrangement is known as a “bistatic” orientation. Finally, the detectionprocessing module determines whether the remote target containsexplosive substances and further locates the target by processing thecollected data from the gamma ray detector, status information collectedfrom the neutron source, and one or more position sensor(s) associatedwith the neutron shield. More specifically, the position sensor(s)associated with the neutron shield transmit the azimuth and elevation ofthe aperture to the detection processing module, which in turndetermines the thermal neutron beam direction and the remote target'slocation.

Embodiments of the present invention include a second, preferablyrotatable, neutron shield defining a second aperture. The two aperturesare each oriented in a different axis, and each neutron shield mayrotate independently at different speeds or remain fixed. Thisarrangement provides further control and fine tuning of the neutronbeam's direction, its scanning speed, and its dimensions.

Embodiments of the present invention include other features, includingbut not limited to neutron amplifiers, neutron focusing elements,neutron beam-forming components, distance or imaging sensors, and apixilated gamma ray detector. The gamma ray detector may be eithermonolithic or contain multiple elements. A detector with a large numberof sensing elements may be described as “pixilated.” The proposedpixilated gamma ray detector is useful to more accurately locate theposition of a hostile target by recognizing the incident angle ofincoming gamma rays.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more readily appreciated when considered in connection with thefollowing detailed description and appended drawings, wherein:

FIG. 1 is a perspective view showing one exemplary embodiment of thesubject invention wherein the apparatus is carried in a land vehiclesuch that the neutron source is supported in a position for scanning asearch area which, in this case, is a roadway having buried therein animprovised explosive device (IED);

FIG. 2 is another simplified depiction of the subject invention whereina vehicular mounted embodiment of the apparatus scans a search area toidentify hostile targets at various elevations from a standoff distance;

FIGS. 3A and 3B show, respectively, forward-looking perspective views asmight be encountered by the driver of a vehicle carrying the apparatusand implementing the method of this invention wherein buildingstructures line the sides of a roadway and a parked vehicle lies ahead,with FIG. 3B depicting in exemplary fashion a scanning path for aneutron beam according to the subject invention with a flash-likeresponse representing the generation of gamma rays which occurs when theneutron beam interacts with substances of interest, e.g., nitrogen, in ahostile target;

FIG. 4 is a schematic representation of the subject apparatus fordetecting remote explosive substances according to one embodiment ofthis invention;

FIG. 5 is a simplified, perspective view showing one possibleconfiguration for the shield and beam forming features of this inventionwherein a first rotatable shield surrounds a neutron source and includesa generally horizontally arranged elongated aperture and a secondrotatable shield, generally spherical in shape, surrounding the first,inner shield whereby the second rotatable shield has an elongated,generally vertically extending aperture which overlaps the aperture inthe first, inner rotatable shield with the overlapping regionestablishing the vector along which a neutron beam projects;

FIG. 6 is an enlarged, fragmentary view of one exemplary embodiment of aposition sensor (or “encoder”) which may be used in connection with thesubject invention to monitor the instantaneous vector of the neutronbeam;

FIG. 7 is a perspective view of an alternative shield configurationshowing a rotatable reticule having an aperture therein to establish theshape and direction of a projected neutron beam;

FIG. 8 is a view as in FIG. 7 showing an alternatively shaped apertureforming the rotatable reticule;

FIGS. 9A and 9B represent simplified, cross-sectional views of theembodiments of FIGS. 7 and 8, wherein FIG. 9A represents the reticule ata position wherein the neutron beam is sealed within the shieldingstructure because the aperture in the reticule has been moved to aclosed position and FIG. 9B represents a condition of the reticulewherein its aperture has been moved to an open position, so as to directa neutron beam from the neutron source along a vector toward the searcharea;

FIG. 10 is an exploded, perspective view showing another embodiment ofthe shielding system, wherein two, independently rotatable reticules aresupported relative to a generally spherical first neutron shield, witheach reticule having an aperture therein;

FIGS. 11A and 11B are simplified, cross-sectional views of the shieldingembodiment of FIG. 10, wherein the two reticules are shown at different,time-displaced positions so that the neutron beam projecting from theneutron source can be controllably directed along a predictable vectortoward the search area;

FIG. 12 is an exploded view of yet another shielding configurationwherein two reticules, independently rotatable, are supported onrespective axes;

FIG. 13 is an assembled view of the embodiment illustrated in FIG. 12showing the manner in which the projected neutron beam can be scannedalong a predictable path by relative movement of the reticules such thatthe overlapping portions of their respective apertures enable adirectional shift in the neutron beam vector;

FIGS. 14A and 14B represent time-shifted examples whereby the neutronbeam can be controlled in its projection by relative movement betweenthe two reticules;

FIG. 15 is an exploded view of yet another alternative embodiment of theshielding configuration wherein two, independently rotatable reticulesare supported along a common axis of rotation;

FIG. 16 is a simplified, cross-sectional view of the embodimentillustrated in FIG. 15;

FIG. 17 is an exploded view of yet another alternative embodiment of theshielding system wherein two, independently rotatable reticules aresupported on independent axes of rotation;

FIG. 18 is a cross-sectional view of the embodiment illustrated in FIG.17; and

FIG. 19 is a simplified, perspective view of the pixilated gamma raydetector of this invention wherein the incident angle of incoming gammarays can be independently detected by a plurality of gamma sensingelements to help spatially locate a target from a distance bycorrelating both energy and trajectory of the incoming gamma rays.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1, 2 and 4 illustrate graphically an apparatus 20 for detectingremote explosive substances in accordance with one embodiment of thepresent invention. (Conventional elements, such as housings, mountings,supports, electrical power supplies, etc. are shown in greatlysimplified form or omitted altogether for ease of illustration.) Theapparatus 20 has a neutron beam generator 22, which directs a neutronbeam 24 across a search area that may contain one or more remotesuspicious targets 26. A target 26 may be generally defined as ahostile, hidden or suspicious object that has the potential to harmpeople or property. In its most common embodiment, a target 26 is animprovised explosive device (IED) or bomb. The apparatus 20 alsoincludes a gamma ray detector 28 and a plurality of data collectionmodules and sensors (described in more detail below), along with adetection processing module 30. These several main components of theapparatus 20 are first broadly described by their sub-components, andthen each sub-component is described in further detail.

Reference herein to “one embodiment,” “an embodiment,” “someembodiments,” or similar formulations, means that a particular feature,structure, operation, or characteristic described in connection withthose embodiments, is included in at least one embodiment of the presentinvention. Thus, the appearances of such phrases or formulations hereinare not necessarily all referring to the same embodiment. Furthermore,various particular features, structures, operations, or characteristicsmay be combined in any suitable manner in one or more embodiments.

The neutron beam generator 22 directs a neutron beam 24 along a vectortowards the search area. As shown schematically in FIG. 4, a fastneutron source 32 is surrounded by an optional neutron amplifier 34,which increases the number of fast neutrons prior to their moderation.The optional neutron amplifier 34 is surrounded by a neutron moderator36, which slows some or all of the fast neutrons to thermal energies. Amovable, preferably rotatable, neutron shield 38 and a second optionalmovable (preferably rotatable) neutron shield 40 enclose a void 42. Theoptional second neutron shield 40 at least partially overlaps the firstneutron shield 38. The neutron moderator 36, optional neutron amplifier34, and the fast neutron source 32 are contained within the void 42.Also located in the void 42 is an optional neutron focusing element 44.Each of the movable neutron shields 38, 40 defines an aperture,apertures 46 and 48 respectively, which cooperate as a beam former todirect the neutron beam 24 along a vector. In other words, the overlapbetween the first 46 and second 48 apertures allows a projected beam 24of neutrons to escape from the generator 22 so that the beam 24 can bescanned across a search area suspected to contain one or more hostiletargets 26. An optional neutron amplifier 50 within the void 42 andimmediately before the overlapped region of the apertures 46, 48 can beused to increase the number of neutrons in the neutron beam 24. Anoptional supplemental neutron beam-forming component 52, situated alonga path of the neutron beam 24, can be used in cooperation with theapertures 46, 48 to further focus the neutron beam 24.

The gamma ray detector 28 is used to detect gamma rays 54 emitted fromthe remote target 26. Preferably, the gamma ray detector 28 is spacedapart from the neutron beam generator 22 by several meters, e.g., threemeters. As shown in FIGS. 2 and 3, substances of interest within theremote target 26 will radiate gamma rays 54 with characteristic emissionspectra when bombarded by neutrons. A portion of these gamma rays 54 areintercepted by a gamma ray spectrometer 56 portion of the gamma raydetector 28. The spectrometer 56 is shielded from nuisance gamma raysoriginating from sources other than the remote target 26 by a gamma rayshield 58.

Neutron source status information is collected from a plurality ofsensors within or near the neutron source 32 and reported via datachannel 60. Furthermore, two position sensors 62 and 64, one for eachshield 38, 40, monitor the instantaneous positions of the respectiveshields 38 and 40, and therefore are capable of discerning the vectorposition or orientation of the neutron beam 24 at any moment in time. Anoptional imaging sensor (e.g., a video camera or its functionalequivalent) 66 may be provided, along with a distance sensor 68, and adetection data collection module 70. The two position sensors 62, 64determine the positions of the two apertures 46, 48, respectively. Eachof the two position sensors 62, 64, the data channel 60, the optionalimaging sensor 66, and the distance sensor 68 collects and transmits itsdata to the detection processing module 30. The detection datacollection module 70 collects and transmits the data from the gamma raydetector 28 to the detection processing module 30. As shown in FIG. 6,the position sensor 62 (and likewise 64) can be of the well-knownencoder-type which may be either separately fitted to some movableportion of either shield 38, 40, or may be incorporated directly intothe motor drive system which controls movement of the respective shields38, 40.

The optional imaging sensor 66 also allows for the system to be switchedoff temporarily, either manually or automatically, if the imaging sensordetects the images of civilians or other sensitive elements in the scenedownrange of the neutron beam. After determining that the area is clearof sensitive elements, the beam can be switched on again, eithermanually or automatically.

The detection processing module 30 processes data, including but notlimited to neutron source status information collected from a pluralityof sensors within the neutron source and reported via data channel 60,position data provided from the two position sensors 62, 64, theoptional imaging sensor 66, the distance sensor 68, and the detectiondata collection module 70. Based on the provided data, the detectionprocessing module 30 determines whether the remote target 26 containsany substances of interest, as well as the location of the remote target26 by inference from the orientation of the beam vector at the moment intime when the gamma ray detector 28 senses the incoming gamma rays 54from the target 26.

As shown in FIG. 4, a compact fast neutron source 32 may be preferredbecause it is portable, simple to construct, and a convenient source ofsignificant neutron flux. Alternative types of such neutron sources 32may be used, however, in appropriate circumstances. For portable fieldoperations, the maximum dimension of the neutron source 32 should beminimized to the extent practical. Numerous types of known fast neutronsources have a maximum dimension smaller than approximately 100 cm, asis desirable here, including but not limited to spontaneous fissionradioisotopes, accelerator-based sources, alpha reactions, photofission,and plasma pinch. Some embodiments have spontaneous fission neutronsources using radioactive isotopes, such as Californium-252. In someembodiments, neutrons are produced by sealed tube or accelerator-basedneutron generators. These generators create neutrons by collidingdeuteron or triton beams into targets containing deuterium or tritium,causing fusion with attendant release of neutrons. Some embodiments havealpha reaction sources, in which alpha particles from alpha-radioactiveisotopes, such as polonium or radium, are directed into targets made oflow-atomic-mass isotopes, such as beryllium, carbon, or oxygen. Anembodiment may also use photofission sources, including beryllium, inwhich gamma rays are directed into nuclei capable of emitting neutronsunder certain conditions. Another kind of neutron source is the plasmapinch neutron source or fusor source, in which a gas containingdeuterium, tritium, or both is squeezed into a small volume plasma,resulting in controlled nuclear fission with attendant release ofneutrons. Pulsed neutron generators using the fusor technique are alsocommercially available.

As shown in FIG. 4, the fast neutron source 32 is surrounded by aconventional neutron amplifier 34, which increases the number of fastneutrons prior to their moderation by the neutron moderator 36. Neutronamplifiers 34 emit more neutrons than they absorb when irradiated byneutrons. Known materials used as neutron amplifiers include, but arenot limited to, thorium, lead, beryllium, americium, andnon-weapons-grade uranium and plutonium. Since the most common neutronamplifiers 34 operate on high energy neutrons, some embodiments mayinclude one or more high energy neutron amplifiers or premoderatoramplifiers, thereby maximizing the number of neutrons in the neutronbeam for a given power dissipation, physical size, cost, and weight.Other types of neutron amplifiers 34 which may be used for thisinvention operate on thermal energy neutrons. Therefore, someembodiments may include a thermal neutron or post-moderator amplifier 50as well.

Because the neutrons produced by the fast neutron source 32 and theoptional premoderator amplification stage 34 have energies tens tohundreds of millions of times larger than the energies required forthermal neutrons in the present apparatus 20, some or all of theneutrons may be slowed down to thermal energies—energies in thermalequilibrium with nominally room temperature surroundings (0.026 eV)—bythe neutron moderator 36. This process is known as neutron moderation orthermalization.

Neutron moderation is conventionally achieved by scattering or collidingthe neutrons elastically off light nuclei that either do not absorb themor else absorb them minimally. Since the light nuclei are of the samerough order of magnitude in mass as the neutrons themselves, eachneutron imparts significant energy to each nucleus with which itcollides, resulting in rapid energy loss by the neutrons. When theneutrons are in thermal equilibrium with their surroundings, a givenneutron is just as likely to get an energy boost from a slightlyfaster-than-average nucleus as it is to lose a slight amount of energyto a slightly slower-than-normal molecule. As a result, thermal neutronsremain thermal. Among the most effective moderator nuclei are deuteriumand carbon-12, since they are light and do not absorb appreciable numberof neutrons. Light hydrogen is also an effective moderator because,although it absorbs a small number of neutrons, its extremely low atomicweight of 1 allows for extremely efficient moderation. Polyethylene,containing carbon and light hydrogen, is thus an effective moderatorcompound.

As shown in FIG. 4, neutron moderation can be achieved by passing fastneutrons emanating from the source 32 through the neutron moderator 36.Some of the optional types of neutron sources mentioned above produceneutron beams (anisotropic sources), while others produce neutrons withtrajectories radiating equally in all directions (isotropic sources).Nevertheless, the effect of moderation, with its numerous elasticscattering events per moderated neutron, yields a fairly isotropicdistribution of neutron trajectories. For this reason, one particularlydesirable shape for the neutron moderator 36 is a hollow sphere with thefast neutron source 32 and the optional premoderator amplifier 34inside. For a deuterium oxide (“heavy water”) moderator 36, thethickness required to moderate nearly 100% of deuterium-deuteriumfusor-source neutrons having energies of the order of 2.45 MeV tothermal energies is of the order of 30 cm; for a graphite moderator, thethickness is greater. See, e.g. G. Friedlander et al, Nuclear andRadiochemistry (3d ed., Wiley and Sons 1981). The actual moderator 36may be thinner than this, if some energetic neutrons are to be left inthe beam 24.

Simply sending thermal neutrons into space in all directions would notallow a target of interest to be located spatially within a search area.For this reason, it is useful to scan the surrounding landscape withneutron beam 24. The neutron beam 24 is formed by enclosing the neutronmoderator 36 with a movable neutron shield 38 having an aperture orreticule 46, which could be variable in both shape and size. The neutronshield 38 could be fabricated of one or more substances known to absorbneutrons, such as boron, lithium, cadmium, hafnium, or gadolinium. Insome embodiments, described in greater detail below, a stationaryneutron shield is filled with one or more rotatable reticules.Embodiments shown in FIGS. 1, 2, 4 and 5 include a second, outer,movable neutron shield 40 defining a second aperture 48. Each neutronshield 38, 40 may move, e.g., rotate or translate, independently atdifferent speeds or remain fixed, or may shift or pivot back-and-forthin response to a predetermined scanning methodology.

FIG. 3A is an exemplary perspective view as may be perceived by a personoperating the subject apparatus 20. In the most practical embodiment ofthis invention, the apparatus 20 is mounted on a mobile carrier 74which, as shown in FIGS. 1 and 2, may take the form of an armored landvehicle. However, other carrier 74 embodiments can be envisioned,including tailored land vehicles, marine vessels, aircraft and the like.In other words, the carrier 74 may comprise any structure capable ofsupporting the neutron source 32 opposite a search area. Thus, in FIG.3A, the perspective view may be that of an area suspected to contain oneor more hostile targets such as bombs or explosive devices which couldbe hidden in any conceivable location below the ground, on the ground orabove the ground. Thus, as the search area is approached, an operator ofthe apparatus 20 upon perceiving the view presented in FIG. 3A, will notbe able to accurately predict where a target 26 may reside. For thisreason, the apparatus 20 is constructed so that the neutron beam 24 canbe scanned across the search area. For example, the circuitous dashedlines in FIG. 3B represent a methodical, serpentine-like back-and-forthscanning of the search area with the neutron beam 24 over a definedperiod of time. In other words, if for example a motor carrier 74 werestationary, the back-and-forth scanning of the search area may take theform illustrated in FIG. 3B. Of course, other scan path methodologiescan be used including up and down, circular, zig-zag or other scanningpatterns as may be deemed appropriate. In these examples, a hostiletarget, e.g., IED, is hidden within a vehicle 80 parked along theroadside in the search area. When the neutron beam 24 scans across thevehicle 80 concealing a target 26, a flash of gamma rays 54 is producedand detected by the gamma ray detector 28. The position sensors 62, 64are effective to specify the orientation of the neutron beam vector atthe moment the gamma rays 54 are detected by the detector 28 so as tolocate the target 26 in the search area.

The void 42 and the neutron focusing element 44 conserve the neutronsand thus maximize the number of neutrons in the neutron beam 24. Thermalneutrons radiate isotropically; that is, they radiate in all directions,into a solid angle of 4π stearadians—the solid angle of a sphere.However, the goal is to produce a fairly narrow thermal neutron beam,e.g. of the order of 0.1 radians wide and 0.3 radians high, orapproximately 0.03 stearadians. As a result, most of the neutrons arewasted—approximately 399 out of 400 of them. Therefore, the presentapparatus may use one or more techniques to focus the neutrons, so as toconserve potentially wasted neutrons. To accommodate such focusing, theapparatus 20 may include the optional void 42, which can be nearly zeroto many centimeters thick. In recent years, numerous schemes forcreating thermal neutron lenses have been described, including but notlimited to capillary optics, silicon lenses, beryllium diffractionlenses, and nickel reflectors. The apparatus 20 may use one or more ofsuch neutron focusing elements 44 to conserve neutrons. The exemplaryfocusing element 44 depicted in FIG. 4 may be a beryllium diffractionlens.

The neutron amplifier 50 at the aperture 46 increases the number ofthermal neutrons in the neutron beam 24. Certain elements, such asthorium, emit more thermal neutrons than they absorb when dosed withthermal neutrons, effectively acting as neutron amplifiers 50.Therefore, the present invention may optionally include a neutronamplifier 50 to further enhance its performance.

The supplemental neutron beam-forming component 52 can be used to focusthe neutron beam 24 more precisely in the event the apertures 46, 48alone do not provide the desired degree of focus. Neutron beam-formingcomponents can be made with materials, such as nickel, that reflectneutrons at very low incident angles. The present invention may includea tubular or other similarly shaped neutron beam-forming component 52 tofurther enhance its performance.

As shown in FIGS. 1-4, the neutron beam 24 is directed toward a searcharea suspected to contain a remote target 26 containing explosivematerials with, in this case, large amounts of nitrogen or othersubstances of interest. Although some of the neutrons are scattered,absorbed, or reflected by air, a significant portion of the neutrons isestimated by simulation to reach a remote target 26 tens of meters away.Neutrons of all energies penetrate, to at least some degree, virtuallyall materials commonly used to shield explosives, including steel,glass, and many materials containing plastics and concrete. Forinstance, to reduce the flux of an incident thermal neutron beam by halfwould require a thickness of approximately 10 cm of steel, 15 cm oflead, 30 cm of aluminum, sand or concrete, 40 cm of glass, or 25 cm ofwater. As a result of the neutron bombardment, gamma rays 54 radiateisotropically from the remote target 26 similar to the illustrations inFIGS. 2 and 3B. These fairly high energy 10.83 MeV gamma rays 54 alsopenetrate, to at least some degree, virtually all materials commonlyfound to shield explosives.

A portion of these gamma rays 54 are intercepted by a gamma ray detector28 fitted with one or more detector elements acting collectively as agamma ray spectrometer 56. The detector 28 and its associated system areconfigured to determine when the gamma rays 54 meet at least onepredetermined condition. These conditions may change from location tolocation and depending on the particular characteristics of the target26. For examples, the predetermined conditions can include energydistribution values, background/nuisance noise characteristics, countrate changes, and angle of incidence for the incoming gamma rays 54. Tostate it more generally, the predetermined condition for which thedetector 28 is watching will involve at least one but more typicallyseveral mathematical formulas, algorithms or comparisons that indicate asubstance of interest in the remote target 26. As but one specificexample, the detector 28 may be configured to determine gamma rays withenergies above a threshold value which, by way of example, could be setat 8 MeV.

The spectrometer 56 is protected from nuisance gamma rays originatingfrom sources other than the remote target 26 by a gamma ray shield 58which could take the form of a collimator, constructed of lead or othergamma ray shielding substances. The gamma ray spectrometer 56 typicallyresolves gamma ray energies in the 8-11 MeV range, i.e., above thethreshold value, with a minimum precision of the gamma ray energy neededto distinguish between return gamma rays 54 from different substances ofinterest. Portable gamma ray spectrometers 56 capable of resolvingenergies at that level and with that precision are often constructed ofeither scintillators, such as sodium iodide (NaI), cesium iodide (CsI)(both with and without various dopants, such as thallium), or variousplastics, or of semiconductors such as high purity germanium (HPGE) orcadmium-zinc-tellurium (CZT), with thicknesses varying from severalmillimeters to several centimeters. However, detectors 28 constructed ofother materials may also be used. The typical method by which gamma raysare detected in these spectrometers is a plurality of pair production,Compton scattering, and conventional scintillation.

The gamma ray detector 28 may employ a spectrometer 56 design composedof a single element, which might be referred to as a “monolithic”detector. However, as shown in FIG. 19, it is possible that the detector28 may be fitted with multiple sensing elements in its spectrometerportion 56 which are capable of independently sensing incoming gammarays 54. This latter type of detector 28 may be referred to as a“pixilated” detector. Gamma rays 54 of the energies of interest, e.g.,including those above 1 MeV, interact with matter in several ways. Eachinteraction way results in a phenomenon that is detectable. At higherenergies, such as 10.83 MeV, one type of typical interaction involvesthe creation of an energetic electron-positron pair. These particlestransit the detector 28, scattering electrons and some nuclei as they doso, in a process known as Rutherford scattering. The Rutherford scatterevents cause low energy photons to be produced. These can be detected byoptical devices such as photo diodes, if they are produced in atransparent medium known as a scintillator. If they are formed in asemiconductor, they can produce varying electrical currents that canalso be sensed. As the positron loses energy it ultimately annihilateswith an electron in the environment, creating two 511 KeV gamma raysthat can be detected by Compton scatter events by either optical orelectronic processes as just described. Therefore, the use of multiple,independently sensed detector elements allow position information to becorrelated to allow additional accuracy in the determination of both theenergy and the trajectory of the incoming gamma rays 54. Therefore, byincluding a plurality of gamma sensing elements within the gamma raydetector 28, each sensing element capable of independently detecting andreporting gamma ray energy levels from a target 26 in a search area, itis possible to more accurately spatially locate the target 26 from asafe distance by correlating both the energy and trajectory of theincoming gamma rays 54.

Pair production consists of the creation of an electron-positron pairfrom the incident gamma ray 54 as it passes the environment of anucleus. In such interactions, the energy of the gamma ray 54 is firstconverted into the rest mass of the electron and the positron (511 KeVeach, for a total of 1.022 MeV), and the surplus gamma ray energy abovethis value is converted to equal amounts of kinetic energy for theelectron and the positron. Thus, for a 10.83 MeV gamma ray, 1.022 MeV isgiven up to the rest mass of the electron-positron pair, and theremaining 9.408 MeV is divided equally between the electron and thepositron, with the result that each has a kinetic energy of 4.704 MeV.The half-angle between the outbound trajectories of the pair isapproximately equal to the ratio between the total energy of the gammaray 54 and the total produced rest mass, expressed in radians. For thecase of a 10.83 MeV gamma ray, this half-angle value is 0.094 radians,or 5.4° of arc, for a total of 10.8° total included angle between theoutbound electron and positron. (The above discussion ignores the verysmall energy and momentum transferred to the nucleus in which the paircreation takes place.) Because they are anti-matter, positronsultimately annihilate with electrons found in the detector, but theannihilation cross-section is extremely small at the high energies ofthis problem. The result is that both the electron and the positron loseenergy by conventional Rutherford scattering with electrons (mostly) andnuclei (occasionally) in the detector, until the energy of the positronis low enough for it to have a non-negligible annihilationcross-section. When it finally annihilates, that event produces a pairof 511 KeV gamma rays that can be detected with a scintillation orsolid-state detector. The energy from incident gamma rays that do notpair-produce, and lose energy via Compton scattering instead, is alsodetected with scintillation counters or solid-state detectors. Therelative number and intensity of the detections allows for computingenergy and flux by summing the energies associated with the eventsdetected by these methods.

Knowledge of the included angle of the created electron-positron pairenables some embodiments to determine the incident angle of a detectedgamma ray by using a pixilated detector 28, which senses the angulardirection of the pair-production track left by the gamma ray asdescribed above and shown in FIG. 19.

The gamma ray spectrometer 56 is preferably spaced, for example, 3meters apart from the thermal neutron source 32 in order to minimize theneutron-irradiated air path seen by the spectrometer 56, therebyreducing background signal. The term for this arrangement is “bistatic”orientation. The spectrometer 56, neutron source 32, and the neutronshielding 38, 40 are typically mounted on a rotatable mast 76 or supporton the carrier vehicle 74.

Simulations show that a thermal neutron beam of 3×10¹⁰ neutrons persecond will detect a 10 kilogram conventional explosive target 20 metersaway in ⅓ of a second with an SNR of 1.5, using an optimal detector 28with a frontal area of ½ meter by ½ meter. The apparatus 20 may work atranges up to 30 meters.

In order to determine whether the remote target 26 contains anyexplosives, the gamma ray detection data collection module 70 collectsand transmits the detected gamma ray data as a function of time from thegamma ray spectrometer 56 to the detection processing module 30. Tofurther, i.e., more accurately, locate the remote target 26, theorientation of the neutron beam 24 is measured simultaneously with thedetermination that gamma rays 54 received at the detector 28 meet thepredetermined condition(s). This can be accomplished by the positionsensors 62, 64 which collect and transmit the positions of the twoapertures 46, 48 as a function of time to the detection processingmodule 30 for further processing. The positions of the apertures 46, 48may be defined by an azimuth and an elevation, and may be used by thedetection processing module 30 to determine the vector of the neutronbeam 24. Also, neutron source status information is collected from aplurality of sensors within or proximate to the neutron source 32 andreported via data channel 60 to the detection processing module 30.

In some embodiments, the detection processing module 30 may determinethe elevation and azimuth of the remote target 26 based on thedetermined vector of the neutron beam 24. Optionally, a radar or othertype of distance sensor 68 may be used to further identify the remotetarget's 26 approximate position based on the estimated distance,elevation, and azimuth. Other types of distance sensors 68 may rely onlaser triangulation, phase angle analysis, acoustic feedback signals,and the like.

Another way to identify the approximate position of a remote target 26is by computing the thermal neutron beam direction (i.e., vectorrelative to the carrier 74) and the incident angle of the detected gammarays 54 as determined by the pixilated detector 28. If the two computedlines do not intersect, then the detected gamma rays 54 are likelynuisance gamma rays originating from sources other than the remotetarget, and the detection processing module 30 may ignore the detectionas a background event. If the two computed lines intersect each other,the intersecting point is the estimated position of the suspicioustarget. In addition, images from the optional imaging sensor 66 may beused by the operator to rule out false detection in situations where theestimated position falls into a region of open space, thereby reducingthe false alarm rate of the apparatus 20. In such situations, thedetected gamma rays 54 are likely attributed to atmospheric nitrogenrather than an object containing explosives.

The detection processing module 30 may also use pattern recognition (viasensor 66) or other techniques to take into consideration other factors,such as time of detection, flight time of the neutrons 24 and gamma rays54, and background noise levels. These data may further be converted totactical decisions according to the user's concept of operations. Imagedata from sensor 66 may be used to monitor for the probability ofsensitive entities in the search area, such as human beings or valuableproperty items that could suffer damage. When such sensitive entitiesare detected or probable, the neutron beam 24 can be altered by varioustechniques to avoid or mitigate harm. For example, the neutron beam 24can be switched off, modulated, redirected or paused when the presenceof a sensitive entity in the search area is probable.

As shown in FIGS. 1 and 2, a detection apparatus 20 as described abovemounted on a mast 76 and projecting neutron beam 24 and receiving gammarays 54 for use may be mounted on a manned or remotely controlledvehicle 74 traveling, e.g., at the head of a convoy to detect IEDs whichcould be hidden on a utility pole 78 or in a parked car 80 (car bomb) atstandoff or near-standoff range—far enough ahead to allow for a traffichalt or evasive maneuvers prior to entering the IED's kill radius.

FIGS. 7-18 show various alternative arrangements for the neutronshielding system so as to achieve a controlled scanning path for theneutron beam 24. FIGS. 7 and 8 show an embodiment where the firstneutron shield 38′ includes a large opening 82 partially covered by aplanar, semi-circular shutter 84. The first neutron shield 38′ caneither be support, such as on a mast 76 for example, for rotationrelative to the carrier 74 or fixed. In this example, it is assumed thatthe first neutron shield 38′ is rotatable as indicated by thedirectional arrow above and by the position sensor 62. The shutter 84 ispreferably fixed (i.e., non-movable) relative to the first shield 38′,and can take many other shapes or forms and have additional holes asneeded to achieve desired results. In these examples, the secondrotatable shield takes the form of a planar reticule 86 whose movementsare monitored by position sensor 88. The reticule 86 includes anaperture 90, the shape of which is shaped and sized to deliver a neutronbeam 24 with a scan speed, angular size, and within scan limits dictatedby the problem addressed by the apparatus. The aperture 90 shape isadaptable or configurable so as to achieve a desired property orperformance requirement that could be specific to each application forthe device. For simplicity, additional shielding to prevent the leakageof neutrons due to the thickness of the reticule 86 is not shown. Itwill be understood that the reticule 86, as well as the shutter 84,could take non-planar shapes without departing from the describedfunctionality.

FIGS. 9A and 9B depict a simplified cross-section taken through eitherof the embodiments of FIG. 7 or 8, wherein a neutron beam 24 is formed,controlled and directed by movement of the reticule 86 relative to thefirst shield 38′. It should be noted, that it is not necessary that thefirst shield 38′ be rotatable, although providing rotation relative tothe carrier 74 will provide added scanning convenience. FIGS. 9A and 9Bare schematic depictions of the single rotating reticule 86 embodimentof FIGS. 7 and 8 showing a neutron beam 24 emanating through theaperture 90 in the rotating reticule 86. FIG. 9A represents the relativeposition of the rotating reticule 86 and non-rotating shutter 84 at timeT1 where the aperture 90 in the rotating reticule 86 is eclipsed by thenon-rotating shutter 84 so that no neutron beam emanates from theapparatus 20. FIG. 9B represents time T2 showing a neutron beam 24emanating through the aperture 90 in the rotating reticule 86.

FIG. 10 shows a pair of co-axially located planar rotating reticules 86,92 without any shutter over the opening 82. In this embodiment, theapertures 90, 94 in the reticules 86, 92 have different shapes, and thedirections of rotation are opposite. The rotation speeds are variable,and may bear no simple relationship to one another, except forfulfilling the requirements of the specific task. Likewise, the shapesand sizes of the apertures 90, 94 in the reticules may be designed todeliver a neutron beam 24 with a scan speed, angular size, and withinscan limits dictated by the problem addressed by the apparatus. Forsimplicity, additional shielding to prevent the leakage of neutrons dueto the thickness of the reticules 86, 92 is not shown. A position sensor96 monitors the instantaneous position of the second reticule 92.

FIGS. 11A and 11B are schematic depictions of the co-axial rotatingreticules 86, 92 embodiment of FIG. 10. FIG. 11A represents the relativepositions of the reticules 86, 92 at time T1 where the overlappingportions of apertures 90, 94 in the two rotating reticules 86, 92 allowa neutron beam 24 to be directed in a general vector away from theapparatus 20. FIG. 11B represents time T2 showing a neutron beam 24emanating through the overlapping portions of apertures 90, 94 in thetwo rotating reticules 86, 92 along a different vector so as toillustrate a sweeping path or motion for the neutron beam 24 betweentimes T1 and T2.

FIGS. 12-14B show a pair of co-planar rotating reticules 86′, 92′. Inthis embodiment, as in the previous example, the apertures 90′, 94′ inthe reticules 86′, 92′ may have different shapes. FIGS. 12 and 13 showapertures 90′, 94′ with similar shapes that are minor images of oneanother. In this configuration, the reticules 86′, 92′ may rotate inopposite directions, although the directions of rotation are variable.Alternatively, the reticules 86′, 92′ can be driven in partial,back-and-forth rotations to achieve a desired scan path. As above, thespeeds are variable, and may bear no simple relationship to one another,except for fulfilling the requirements of the specific task. The shapesand sizes of the apertures 90′, 94′ can be designed to deliver a neutronbeam 24 with a scan speed, angular size, and within scan limits dictatedby the problem addressed by the apparatus.

FIGS. 14A and 14B are schematic depictions of the co-planar rotatingreticules 86′, 92′ embodiment of FIGS. 12-13. FIG. 14A represents therelative positions of the reticules 86′, 92′ at time T1 where there areno overlapping portions of apertures 90′, 94′ in the two rotatingreticules 86′, 92′ such that no neutron beam emanates from the apparatusat time T1. FIG. 14B represents a later time T2, showing a neutron beam24 emanating through the overlapping region of the apertures 90′, 94′.

FIG. 15 is an exploded depiction of the co-axial rotating reticules 86,92 combined with a non-rotating shutter 84 where the overlappingportions of apertures 90, 94 in the two rotating reticules 86, 92 allowa neutron beam 24 to be directed in a general vector away from theapparatus 20, as shown in FIG. 16. FIGS. 17 and 18 show yet anothervariation wherein the axially offset rotational axes of the reticules86′, 92′ are combined with a non-rotating shutter 84.

Although the various views have shown only one aperture slot in eachreticule, it will be understood that arrays of such apertures can bearranged in increments about either or both of the reticules, so as toaccomplish any of a multiplicity of neutron beam 24 sweep patterns, suchas, but not limited to, continuous back-and-forth scanning, with one ormore sweeping beams, repeating side-to-side scanning with the scanalways moving in the same direction, azimuthal scanning at successivelydifferent elevation angles, and other such combinations of scan.

The neutron source 32 emits neutrons in all directions. In theembodiments shown, most of the emitted neutrons are stopped by theshielding that immediately surrounds the source 32. Neutrons not stoppedby this shielding are emitted toward the apertures 46, 48, 90, 90′, 94and 94′ and comprise the primary beam forming features of the apparatus20.

Thus, neutrons encountering the surrounding shielding are stopped by it,except for those that pass through the apertures. These neutronsconstitute a beam 24. Since the shield and/or reticule is rotating, thebeam 24 will rotate with it, thereby enabling a scanning motion.

The apparatus 20 and methods for carrying out the invention utilize atleast one and optionally several moving reticule(s) and one or moreoptional fixed shutters 84 made of suitable neutron shielding material.The reticule(s) cooperate(s) with the permanent shield that surroundsthe neutron source 32 and thereby collectively control all (orsubstantially all) the emission of neutrons from the apparatus 20. Eachmovable shield or reticule includes an aperture formed therein. Theaperture may also include a wedge-shaped cut that goes all the way tothe reticule's outer edge, or any other relevant shape designed tosuitably control the eminating beam 24. At least one of the reticulesmoves, or more preferably rotates, relative to the shield or otherreticule(s) so that their apertures variably or periodically overlapallowing a neutron beam to be discharged advantageously along acontrollable vector. By thoughtfully shaping and orienting theapertures, and controlling the rotational speed(s) of the reticule(s), adischarged neutron beam 24 can be made to scan or translate apredictable path that is useful for sweeping an area of interest, assuggested in FIGS. 3A and 3B.

In cases where a neutron source 32 is used whose neutron flux has thecapability of being modulated, for example, electronically, suchmodulation capability may optionally be employed to further refine thespecific path of the produced neutron beam. For example, if mechanicalconstraints dictate that an otherwise desirable neutron beam scanningpath would have some undesirable path segments, then a neutron sourcecapable of modulation could simply either be switched off or else set toa low value during those path segments. Information as to when theneutron source should be modulated would be obtained from the positionsensors 62 and 64.

The foregoing invention has been described in accordance with therelevant legal standards, thus the description is exemplary rather thanlimiting in nature. Variations and modifications to the disclosedembodiment may become apparent to those skilled in the art and fallwithin the scope of the invention.

1. An apparatus for locating concealed, hostile targets at a remotedistance by inducing then detecting gamma rays from the target, saidapparatus comprising: a neutron source for producing a neutron beamcapable of generating gamma rays upon interaction with a target; acarrier for supporting said neutron source opposite a search area; abeam former for directing a neutron beam from said neutron source alonga vector toward the search area; a moveable connection operativelyassociated with said beam former for moveably scanning the neutron beamvector relative to said carrier across a wide search area; a gamma raydetector for detecting gamma rays emanating from a target in the searcharea; at least one position sensor associated with said beam former formonitoring the instantaneous vector of the neutron beam relative to saidcarrier; and a detection processing module operatively coupled to saidsensor and said gamma ray detector for associating the instantaneousvector of the neutron beam relative to said carrier at the moment saidgamma ray detector detects gamma rays from a target in the search areato spatially locate the target from a remote distance.
 2. The apparatusof claim 1, further including a first neutron shield at least partiallycovering said neutron source for controlling the escape of neutrons. 3.The apparatus of claim 2, wherein said beam former comprises at leastone aperture formed directly in said first neutron shield, said firstneutron shield is rotatable relative to said carrier, and said positionsensor is operatively associated with said first neutron shield.
 4. Theapparatus of claim 3, further including a second neutron shield at leastpartially overlapping said first neutron shield, said second neutronshield having a second aperture.
 5. The apparatus of claim 4, whereinsaid second neutron shield is rotatable relative to said first neutronshield.
 6. The apparatus of claim 4, further including a second positionsensor associated with said second neutron shield.
 7. The apparatus ofclaim 6, wherein said first neutron shield is supported for rotationabout an axis and said second neutron shield is supported for rotationabout a different axis that is maintained generally parallel to therotational axis of said first neutron shield.
 8. The apparatus of claim1, wherein said gamma ray detector includes a plurality of pixelsconfigured to determine an incident angle of a detected gamma ray. 9.The apparatus of claim 1, further including at least one status sensorproximate said neutron source, said status sensor operatively coupled tosaid detection processing module and configured to report the status ofsaid neutron source to said detection processing module.
 10. Theapparatus of claim 1, wherein said carrier includes a land vehicle. 11.The apparatus of claim 1, further including a first neutron shield atleast partially covering said neutron source for controlling the escapeof neutrons, said beam former including at least one reticule rotatablysupported relative to said first neutron shield, said reticule having atleast one aperture disposed therein.
 12. The apparatus of claim 11,wherein said reticule is generally planar.
 13. The apparatus of claim11, wherein said beam former including at least two reticulesindependently rotatably supported relative to said first neutron shield,each of said reticules having at least one aperture disposed therein.14. The apparatus of claim 13, wherein said at least two reticules aresupported for rotation relative to said first neutron shield about acommon axis.
 15. The apparatus of claim 13, wherein said at least twoplanar reticules are supported for rotation relative to said firstneutron shield about respective parallel axes.
 16. A method for locatingconcealed, hostile targets at a remote distance by inducing thendetecting gamma rays from the target, comprising the steps of:projecting a beam of neutrons along a vector toward a search area;producing gamma rays by the interaction of the projected neutron beamwith a target in the search area; monitoring for gamma rays with a gammaray detector; scanning the neutron beam across the search area;determining when the gamma rays monitored by the gamma ray detector meetat least one predetermined condition; measuring the orientation of theneutron beam vector simultaneously with said determining step; andlocating the target in the search area by reference to the neutron beamvector oriented during said measuring step.
 17. The method of claim 16,wherein said step of scanning the neutron beam includes directing theneutron beam in a reciprocating path.
 18. The method of claim 16,further including providing a neutron source and a first neutron shieldat least partially surrounding the neutron source, the first neutronshield having an aperture formed directly therein, and wherein said stepof scanning the neutron beam includes rotating the first neutron shield.19. The method of claim 18, further including providing a second neutronshield at least partially overlapping the first neutron shield, andwherein said step of scanning the neutron beam includes rotating thesecond neutron shield.
 20. The method of claim 19, wherein the step ofmeasuring the orientation of the neutron beam vector includes definingthe respective rotational positions of the apertures in the first andsecond neutron shields.
 21. The method of claim 16, further includingproviding a reticule having an aperture formed directly therein, andwherein said step of scanning the neutron beam includes rotating thereticule.
 22. The method of claim 16, further including providing atleast two reticules having respective apertures formed directly therein,and wherein said step of scanning the neutron beam includes rotating atleast one of the reticules relative to the other reticule so that theirrespective apertures intermittently overlie one another.
 23. The methodof claim 22, wherein said step of rotating at least one of the reticulesincludes rotation both reticules about a common axis of rotation. 24.The method of claim 22, wherein said step of rotating at least one ofthe reticules includes rotating both reticules about a respective axesof rotation parallel to one another.
 25. The method of claim 16, whereinthe step of monitoring for gamma rays includes determining the incomingincident angle of the detected gamma rays.
 26. The method of claim 16,further including providing a neutron source, and sensing the status theneutron source during said step of projecting a beam of neutrons. 27.The method of claim 16, wherein said step of monitoring for gamma raysincludes determining the incoming energy of the detected gamma rays. 28.The method of claim 16, further including the step of specifying thedistance between the target and the gamma ray detector with a distancesensor.
 29. The method of claim 16, further including the step ofvisually inspecting a search area with an imaging sensor.
 30. The methodof claim 16, further including the step of separating the gamma raydetector from the neutron source by a distance of at least 1 meter tominimize gamma ray sensing from nuisance sources associated with thetransmission of neutrons through the air or with the neutron sourceitself.
 31. The method of claim 16, further including the step ofaltering the neutron beam in response to the detection of a sensitiveentity in the scanning area, said altering step including at least oneof temporarily interrupting the neutron beam, redirecting the neutronbeam, modulating the neutron beam and/or pausing the neutron beam.